The overall objective of the proposed research is to elucidate further the influence of thyroid hormone on skeletal muscle growth in chickens. The effect of thyroid hormone on the growth of two specific skeletal muscles will be studied during the second half of embryogenesis and during the first eight weeks posthatching. Our experimental design is based on previous observations in this laboratory which indicate that thyroid hormone has different effects at different stages of muscle growth and that the gastrocnemius is more responsive than the sartorius muscle to severe hypothyroidism posthatching. We will focus on the influence of thyroid hormone on aspects of transcription and translation during the synthesis of proteins which underlie muscle growth. Transcription and translation will be assessed by measuring RNA polymerase activity, polysome sedimentation patterns and the synthetic activity of myosinsynthesizing polysomes. Our studies will be aided greatly by recent investigations of these parameters in normally developing chick skeletal muscle. Circulating levels of thyroid hormone will be assessed during the periods of thyroid hormone deprivation and replacement so that a more accurate interpretation of our data on muscle growth will be possible. Since thyroid hormone clearly plays an important role in muscle growth and development, one objective of the proposed research is to assess the influence of thyroid hormone on skeletal muscle growth in a strain of chickens with hereditary muscular dystrophy.